Document the particle structure

[TimRoith]

In commit bc69cd0 we introduced that a particle has the dimension

$$M\times N \times d$$

where

• $M$ denotes the number of independent runs,
• $N$ is the number of particles and
• $d$ is the dimension.

Also in this commit we dropped the primary support for parallel runs via multiprocessing. This might be reintroduced later.
Furthermore this development changed the structure of the batching routine, this needs to be explained.